Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 5.0	$ 27.9
Accounts receivable, less allowance for doubtful accounts	415.0	482.8
Production fuel, at weighted average cost	68.8	72.3
Gas stored underground, at weighted average cost	21.9	44.5
Materials and supplies, at weighted average cost	109.4	105.6
Regulatory assets	71.9	84.3
Other	113.9	87.7
Total current assets	805.9	905.1
Property, plant and equipment, net	11,695.7	11,234.5
Investments:
ATC Holdings	283.0	274.2
Other	137.5	121.9
Total investments	420.5	396.1
Other assets:
Regulatory assets	1,606.5	1,582.4
Deferred charges and other	86.4	69.7
Total other assets	1,692.9	1,652.1
Total assets	14,615.0	14,187.8
Current liabilities:
Current maturities of long-term debt	356.1	855.7
Commercial paper	82.5	320.2
Other short-term borrowings	0.0	95.0
Accounts payable	431.1	477.3
Regulatory liabilities	139.8	140.0
Other	247.9	260.8
Total current liabilities	1,257.4	2,149.0
Long-term debt, net (excluding current portion)	5,127.5	4,010.6
Other liabilities:
Deferred tax liabilities	1,507.2	1,478.4
Regulatory liabilities	1,373.1	1,357.2
Pension and other benefit obligations	488.7	504.0
Other	310.4	306.4
Total other liabilities	3,679.4	3,646.0
Commitments and contingencies (Note 13)
Common equity:
Common stock	2.3	2.3
Additional paid-in capital	1,947.2	1,845.5
Retained earnings	2,412.5	2,346.0
Accumulated other comprehensive income (loss)	0.1	(0.5)
Shares in deferred compensation trust - 465,450 and 463,365 shares at a weighted average cost of $24.47 and $23.91 per share	(11.4)	(11.1)
Total common equity	4,350.7	4,182.2
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	4,550.7	4,382.2
Total liabilities and equity	14,615.0	14,187.8
IPL [Member]
Current assets:
Cash and cash equivalents	1.6	3.6
Accounts receivable, less allowance for doubtful accounts	237.2	264.9
Production fuel, at weighted average cost	51.4	52.4
Gas stored underground, at weighted average cost	10.0	20.3
Materials and supplies, at weighted average cost	60.8	60.6
Regulatory assets	39.8	41.9
Other	29.6	32.3
Total current assets	430.4	476.0
Property, plant and equipment, net	6,193.3	5,926.2
Other assets:
Regulatory assets	1,209.4	1,189.7
Deferred charges and other	16.6	14.1
Total other assets	1,226.0	1,203.8
Total assets	7,849.7	7,606.0
Current liabilities:
Current maturities of long-term debt	350.0	350.0
Commercial paper	0.0
Accounts payable	203.4	220.3
Regulatory liabilities	92.1	69.7
Other	170.2	187.7
Total current liabilities	815.7	827.7
Long-term debt, net (excluding current portion)	2,181.8	2,056.0
Other liabilities:
Deferred tax liabilities	902.9	910.7
Regulatory liabilities	682.7	685.7
Pension and other benefit obligations	170.3	173.8
Other	242.2	242.4
Total other liabilities	1,998.1	2,012.6
Commitments and contingencies (Note 13)
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	1,927.8	1,797.8
Retained earnings	692.9	678.5
Total common equity	2,654.1	2,509.7
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	2,854.1	2,709.7
Total liabilities and equity	7,849.7	7,606.0
WPL [Member]
Current assets:
Cash and cash equivalents	2.5	23.1
Accounts receivable, less allowance for doubtful accounts	170.8	212.2
Production fuel, at weighted average cost	17.4	19.9
Gas stored underground, at weighted average cost	11.9	24.2
Materials and supplies, at weighted average cost	45.2	42.1
Regulatory assets	32.1	42.4
Other	71.8	54.7
Total current assets	351.7	418.6
Property, plant and equipment, net	5,118.2	4,917.9
Other assets:
Regulatory assets	397.1	392.7
Deferred charges and other	43.5	27.3
Total other assets	440.6	420.0
Total assets	5,910.5	5,756.5
Current liabilities:
Commercial paper	26.4	25.0
Accounts payable	173.6	201.7
Regulatory liabilities	47.7	70.3
Other	95.4	99.2
Total current liabilities	343.1	396.2
Long-term debt, net (excluding current portion)	1,834.1	1,833.4
Other liabilities:
Deferred tax liabilities	540.0	522.4
Regulatory liabilities	690.4	671.5
Capital lease obligations - Sheboygan Falls Energy Facility	63.9	70.2
Pension and other benefit obligations	209.0	213.7
Other	174.8	167.6
Total other liabilities	1,678.1	1,645.4
Commitments and contingencies (Note 13)
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	1,259.0	1,109.0
Retained earnings	730.0	706.3
Total common equity	2,055.2	1,881.5
Total liabilities and equity	$ 5,910.5	$ 5,756.5